Open-cell ceramic foams are commercially desirable for use in a variety of products including filters for molten metals, diesel particulate traps, catalytic converters for automotive exhaust treatment, heat exchangers, heating elements, thermal and electric insulators, etc. In addition to the outstanding high-temperature and chemical resistance afforded by the ceramic, and the benefit of the foam's high degree of porosity and large surface area for such products as filters and catalyst supports, the high strength-to-weight ratio achievable with ceramic foams is an attractive advantage in components for automobiles, aircraft, etc.
In the conventional manufacture of ceramic bodies, including ceramic foams, a number of process steps are required, e.g., grinding, sizing, consolidating, sintering, machining, etc. At each step, inhomogeneities and impurities can be introduced, which may have a deleterious effect on the end product. Another important criterion, which is not feasible with conventional processing, is the ability to manufacture such ceramic bodies in near-final configuration (near net shape), including intricate shapes.
As is described in U.S. Pat. No. 3,947,363, granted Mar. 30, 1976, to M. J. Pryor et al., open-cell ceramic foams can be prepared from an open-cell, hydrophilic flexible organic foam material having a plurality of interconnected voids surrounded by a web of the foam material. The organic foam material is impregnated with an aqueous ceramic slurry so that the web is coated, and the voids are filled, with slurry. Then the slurry-impregnated material is compressed to expel 20-75% of the slurry, and the pressure thereafter is released so that the web remains coated with slurry. After drying, the material is heated, first to burn out the flexible organic foam and then to sinter the ceramic coating, thereby leaving a consolidated ceramic foam having a plurality of interconnected voids surrounded by a web of bonded or fused ceramic in the configuration of the precursor organic foam. With this organic-to-ceramic technique, it has been reported to be necessary to take steps to overcome problems of product non-uniformity caused by a non-uniform distribution of slurry resulting when the organic foam body is compressed by passage through rolls, by excessive handling of organic foam slabs, etc.
In a variation of the above-described process set forth in U.S. Pat. No. 4,076,888, granted Feb. 28, 1978, to G. Perugini et al., a metal, metal/ceramic, and/or ceramic coating is applied to a foamed polyurethane sponge via an electroconducting film (e.g., electroless nickel or copper plating) previously applied to the sponge. A galvanic coating is described as being applied over the electroconducting film, followed by a molten-spray coating of metal/ceramic or ceramic applied by means of a 10,000.degree.-15,000.degree. C. argon plasma flame. The final web is described as being hollow and multi-layered in structure and gradually changing from metallic on the inside to an external ceramic character. The molten-spray coating technique is stated to be applicable only to sponges no thicker than 12 mm if the spray can be applied on one face only, and no thicker than 25 mm if spraying can be applied to two opposite sides.
About ten years earlier, U.S. Pat. Nos. 3,255,027 (granted June 7, 1966, to H. Talsma), 3,473,938 (granted Oct. 21, 1969, to R. E. Oberlin), and 3,473,987 (granted Oct. 21, 1969, to D. M. Sowards) had disclosed a method of making thin-walled alumina-containing structures by firing, in an oxygen-containing atmosphere, thin aluminum sections (such as cans, tubes, boxes, arrays of tubes, honeycombs, etc., or crumpled forms, made from aluminum sheets or formed by extrusion methods), coated with an oxide of an alkali metal, alkaline earth metal, vanadium, chromium, molybdenum, tungsten, copper, silver, zinc, antimony, or bismuth, or precursor thereof as a fluxing agent, and, optionally, a particulate filler refractory. As applied to honeycomb structures, the process was reported to form double-walled sections of refractory having a sheet-like void near the center, said to have resulted from the migration of molten aluminum through fissures in the oxide film formed on the surface of the metal. Oberlin showed this structure to be weak, and described eliminating the double-walled structure by using a vanadium compound and a silicate fluxing agent in the process. Sowards pre-coated the structure with aluminum powder prior to firing, thereby producing the double-walled structure with thicker walls. Only aluminum template structures fabricated from sheets or formed by extrusion were contemplated.
As is stated by Talsma, the interconnected walls of a honeycomb define closed cells or channels longitudinally extending the entire length of the walls. The channels are aligned so as to be parallel to a single common axis, a structure which is less useful for certain purposes than that of open-cell foams, in which the cellular structure is three-dimensional. For example, the particulate collecting efficiency of a ceramic honeycomb filter disposed in an exhaust passage of a diesel engine has been reported to be low (U.S. Pat. No. 4,540,535), and honeycomb catalyst supports have been reported as suffering from a relatively low geometric area and undesirably low turbulence (U.S. Pat. No. 3,972,834).
Co-pending and co-assigned U.S. patent application Ser. No. 818,943, filed on Jan. 15, 1986, now U.S. Pat. No. 4,713,360, issued on Dec. 15, 1987, in the name of Newkirk et al., describes a generic process for producing ceramic products by the directed oxidation of molten precursor metal. In this process, an oxidation reaction product forms initially on the surface of a body of molten parent metal exposed to an oxidant, and then grows from that surface by transport of additional molten metal through the oxidation reaction product where it reacts with the oxidant. The process may be enhanced by the use of alloyed dopants such as in the case of an aluminum parent metal oxidized in air. This method was improved by the use of external dopants applied to the surface of the precursor metal as disclosed in co-pending and co-assigned U.S. patent application Ser. No. 220,935, filed June 23, 1988, now U.S. Pat. No. 4,853,352, issued on Aug. 1, 1989, which is a Continuation of Application Ser. No. 822,999, filed Jan. 17, 1986, in the name of Newkirk et al. In this context, oxidation has been considered in its broadest sense, to mean one or more metals giving up electrons to, or sharing electrons with, another element or combination of elements to form a compound. Accordingly, the term "oxidant" denotes an electron acceptor or sharer.
In the process described in co-pending, co-assigned U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, now U.S. Pat. No. 4,851,375, issued on July 25, 1989, by Newkirk et al., ceramic composite products are produced by growing a ceramic product in a bed of filler material adjacent to a body of molten parent metal. The molten metal reacts with a gaseous oxidant, such as oxygen, which has been allowed to permeate the filler bed. The resultant oxidation reaction product, e.g. alumina, can grow into and through the mass of filler as molten parent metal is drawn continuously through fresh oxidation reaction product. The filler particles are embedded within the polycrystalline ceramic product comprising the oxidation reaction product interconnected in three dimensions.
Another co-pending, co-assigned U.S. patent application Ser. No. 823,542, filed Jan. 27, 1986, now U.S. Pat. No. 4,828,785, issued on May 9, 1989 by Newkirk et al., describes a method of making ceramic composite articles, including tubes, by growing a ceramic product in a permeable bed of filler material which surrounds a mold or pattern of parent metal defining a shape to be inversely replicated as a cavity in the ceramic composite article. The metal mold (e.g., a shaped aluminum rod), embedded in the filler (e.g., a permeable mass of alumina or silicon carbide particles), becomes molten, and the molten parent metal reacts with an oxidant such as oxygen, which has been allowed to permeate the adjacent filler bed. The resultant oxidation reaction product, e.g. alumina, can grow into and through the mass of filler as molten parent metal is drawn through fresh oxidation reaction product. When the molten metal in the space originally occupied by the metal mold has been consumed, there remains a cavity that inversely replicates the shape or geometry of the original metal mold, with the cavity being surrounded by the resulting ceramic composite.
These oxidation reaction processes by directed oxidation provide a diversity of shaped ceramic articles, but heretofore have not been applied to producing rigid ceramic foams. Ceramic foams have a distinctive physical structure which imparts many beneficial properties and uses thereto. This structure is characterized by open cells or channels interconnected randomly in three dimensions, affording a high surface area per unit volume and high strength-to-weight ratio. Turbulent fluid flow results from these three-dimensional cellular structures, which can be advantageous in some applications, and is in contrast to the laminar flow in a honeycomb. A need exists for improvements in such products, and in methods of making them.